The present invention relates to an improved method for the preparation of vinyl chloride polymers or, in particular, to an improved method for adding a polymerization initiator to the polymerization mixture in the suspension polymerization of vinyl chloride monomer in an aqueous medium.
A variety of polymerization initiators hitherto known and employed for the polymerization of vinyl chloride include several peroxy dicarbonates, such as diisopropylperoxy dicarbonate, dicyclohexylperoxy dicarbonate, bis(tert-butylcyclohexyl)peroxy dicarbonate and the like; several peresters, such as tert-butylperoxy neodecanoate, tert-amylperoxy neodecanoate and the like; several diacyl peroxides, such as benzoyl peroxide and the like; and several azo or nitrile compounds, such as azobisisobutyronitrile, dimethylvaleronitrile, 4-methoxy-dimethylvaleronitrile, trimethylvaleronitrile and the like, as well as acetylcyclohexylsulfonyl peroxide and others.
These polymerization initiators have different influences on the productivity of the vinyl chloride polymerization and the quality of the resulting polymers. Extensive investigations have been undertaken for each of the polymerization initiators with the object to establish more advantageous manner of application in a commercial scale, along with the knowledge that the conditions under which the polymerization initiators are added to the polymerization mixture, for example, the temperature of the polymerization mixture and the duration over which the polymerization initiators are added to the polymerization mixture, give different effects to them, even to one and the same polymerization initiator.
It has been a recent tendency, for example, that larger and larger polymerization reactors are used in order to obtain improved productivity as well as the stabilization of the qualities of vinyl chloride polymers. Usually in the polymerization using a large polymerization reactor, water forming the aqueous polymerization medium is heated prior to introduction to the reactor in order to shorten the period of time necessary for the elevation of temperature immediately after completion of introduction of all ingredients into the reactor and hence quicken the initiation of the polymerization reaction. In this procedure using the pre-heated water, not all of the above-mentioned polymerization initiators can be used, but a limited member of them are applicable.
In explanation, most of the polymerization initiators having relatively high activity and applicable to the preparation of vinyl chloride polymers are liquid at room temperature, and must be stored at a low temperature, usually below 0.degree. C., in order to avoid decomposition. When such a polymerization initiator is added to the pre-heated aqueous medium in the polymerization reactor, followed by introduction of vinyl chloride monomer, the premature decomposition of the polymerization initiator takes place before it is uniformly dispersed and dissolved into the vinyl chloride monomer, bringing about localized heterogeneous polymerization to the vinyl chloride monomer dispersed in the aqueous medium as fine droplets. As a result, the polymer products have an undesirable wide particle size distribution and a remarkbly increased number of fish-eyes in the sheets fabricated therewith.
Trials have been undertaken to mitigate the above-described adverse effects. For example, the polymerization initiator is added to the polymerization mixture under vigorous agitation after the water and the vinyl chloride monomer have been introduced into the polymerization reactor, but no satisfactory results can be obtained due to decomposition reaction occurring to the initiator in the aqueous medium at an elevated temperature before it is dissolved into the monomer phase. As a further example, the polymerization initiator is dissolved in advance in the vinyl chloride monomer, and the initiator-containing vinyl chloride monomer is added to and dispersed in the pre-heated aqueous medium, but this procedure also fails to have satisfactory results due to localized polymerization reaction taking place before the monomer has been uniformly dispersed. In any way, the disadvantages encountered in the prior art techniques have hardly been avoided.
When the polymerization of vinyl chloride is conducted in a relatively small polymerization reactor, the preliminary heating of water as the aqueous medium is not always undertaken, but it is usual that all ingredients are introduced into the polymerization reactor without preliminary heating. Even in such procedure of polymerization, in which the steps of introducing water at room temperature, introducing the polymerization initiator, evacuating the reactor and then introducing the vinyl chloride monomer are successively taken, the decomposition reaction of the polymerization initiator may take place before the introduction of the vinyl chloride monomer is finished, if the polymerization initiator is a liquid one at room temperature, such as acetylcyclohexylsulfonyl peroxide or diisopropylperoxy dicarbonate, leading consequently to disadvantageous results similar to the case where the water is preliminarily heated.
It has been a matter of concern in recent years that, polymerization initiators which are not liquid but solid at room temperature should be used for the polymerization of vinyl chloride, such solid initiators being exemplified by azo compounds, diacetylperoxy dicarbonate, bis-tert-butyl-cyclohexylperoxy dicarbonate and the like. The solid polymerization initiators in general are obtained in the form of lumps, coarse particles or flakes, and have a lower rate of dissolution in vinyl chloride monomer than liquid initiators. Therefore, it has been found difficult to homogeneously dissolve the solid initiators into fine vinyl chloride monomer droplets, resulting in uneven concentration of the initiator within or among the monomer droplets, with the above-discussed disadvantages being hardly eliminated.
It has been proposed that the solid polymerization initiator should be added to the polymerization mixture in the polymerization reactor as dissolved in the vinyl chloride monomer in advance prior to introduction. However, the solid polymerization initiators have activities toward decomposition very much enhanced when dissolved in the monomer to the same extent as is witnessed in the case of the aforementioned highly active liquid polymerization initiators and, hence, the advantages of the solid initiators are almost entirely lost.
In addition, some attempts in which the solid polymerization initiators is added to the polymerization mixture and dissolved in the vinyl chloride monomer at room temperature under vigorous agitation have been found unsuccessful. This is because the polymerization reaction of the monomer begins locally at the points where the polymerization initiator dissolves into the monomer droplets, and a very inhomogeneous polymerization reaction takes place in an extent much higher than in the case wherein the highly active liquid polymerization initiators are used, resulting to bring about the disadvantageous broadened particle size distribution and the increased number of fish-eyes to the resultant polymer products.
In conclusion, there have been discovered no satisfactory techniques for overcoming the disadvantages owned by the relatively highly active polymerization initiators and, on the other hand, for taking advantage of the features of such initiators in the polymerization of vinyl chloride in an aqueous medium.